Heating metallic material for subhalide refining



Oct. 10, 1967 v. At BRAUNWARTH ETAL 3,346,368 I HEATING METALLIC MATERIAL FOR SUBHALIDE REFINING Filed Feb. 2, 1965 IMPURE AI ALLOY /n van fors 25 l//c/or A. mun war/f1 //omey United States Patent O 3,346,368 HEATING METALLIC MATERIAL FOR SUBHALIDE REFINING Victor A. Braunwarth, Alexander Gordon Nickle, and

Norman W. F. Phillips, Arvida, Quebec, Canada, assignors to Aluminium Laboratories Limited, Montreal, Quebec, Canada, a corporation of Canada Filed Feb. 2, 1965, Ser. No. 429,826 21 Claims. (Cl. 75-68) This invention relates to the subhalide refining of aluminum wherein impure aluminum-containing material in particulate form is treated with normal aluminum halide gas, such as aluminum trichloride or aluminum tribromide, `at high temperature for conversion of the aluminum in the ycharge to gaseous aluminum monohalide which is thereafter decomposed to yield the purified aluminum metal and restored normal halide. More particularly, the invention is directed to improved procedure and apparatus in etfectuating the described conversion of aluminum in the charge material to monohalide, in operations where the heat for the reaction is supplied by electrical resistance heating, i.e. by passing electric current through the particulate material in the converter. In a more specific sense, the present improvements are concerned with preheating the charge material prior to its introduction to the converting region, e.g., preheating the continuing 'supply of such material so that when it reaches the zone of electrical heating in the converter it is already characterized by a relatively high temperature.

As explained in U.S. Patent No. 2,937,082, of A. H. Johnstone et al., granted May 17, 1960', it is highly desirable to use electrical resistance heating to furnish heat energy `for the endothermic reaction in the converter, a preferred mode of operation being to maintain a large, slowly downwardly moving column of the aluminumcontaining material while supplying electric current between upper and lower electrode means exposed to the charge. Such material is contemplated to be a crude alloy, 'such as so-called carbothermic -alloy derived by thermal reduction of aluminum ore, e.g. bauxite, with carbon as in an electric furnace, and is thus ordinarily a solid metallic composition containing aluminum (in proportion ranging downward and upward from 50%) with other elements such as iron and silicon, and usually some titanium and carbon, or further or other metals, depending on the composition of the original ore. As will be understood, this material is supplied in particulate form, i.e. granular or as lumps or the like, such that it may be appropriately handled as a column or bed yet with sufficient interstices and exposed surfaces of the particles for efficient passage and reaction of the halide gas, eg. aluminum trichloride. Suitable temperatures of operation in the converter are usually of the order of 1000 C. and above and preferably about l200 C. to 1300" C. or somewhat higher.

As explained in the above ci-ted U.S. Patent No. 2,937,- 082, difficulty is encountered when it is attempted to supply relatively cold charge material of the above sort to the converter, either initially or as substantial accretions to the charge, for example by continuous `feed thereto. Aluminum-containing alloys of the stated character (whether obtained by the above so-called carbothermic reduction or otherwise), to which the subhalide distillation process is generally considered applicable, have a negative temperature coeiiicient of resistivity (usually in the solid itself (-and always as beds of granular solid) over a considerable range of elevated temperatures. Thus while beds of alloys of this type may have a relatively high resistance at room temperatures and somewhat above, the resistivity drops at higher temperatures, to quite a low value at about 1000 C., and the problem of 2 electrically heating such beds is correspondingly acute through the indicated range, especially up to about 700 C.

Under these circumstances it is found that when the converter is attempted to 'be operated partly or Wholly with relatively cool alloy, there is a serious and highly objectionable tendency for the particles to fuse or sinter together along specific but unpredictable paths, as 'a result of extreme, localized overheating. These effects occur in consequence, it is believed, of the described negative temperature coefiicient of resistivity, in that the electric current tends to channel in such paths, and the result is not only inefficient heating operation and correspondingly ineilicient reaction but also involves (because of increasing fusion or ultimate solidication of fused junctions, or both) an impairment of desired movement. of material or of the bed downward, and interference with the removal of the spent charge, and indeed with the passage and accessibility of gas to the material.

In consequence, it has been conceived as in the stated patent, that the supply of fresh alloy should be preheated to temperatures near or at the desired reaction values, by other than electrical resistance procedure, i.e. by heat supplied from sources external to the particlesy themselves. If the body of material in the converting region is first established at a suitably high temperature, substantially in all its parts, and additions thereto are likewise made at such tem-perature, e.g. 1000 C. or above, further supply of heat for effectuating the reaction in this region is readily achieved by passage of electric current through the bed or body which then has a relatively low resistance, substantially uniformly throughout.

As indicated, however, attempts to use current for internally heating static beds of granular, fusible metallic material of this sort, or for internally heating such beds, columns or bodies in progress through a converting region, from rather low temperatures up to values of the order last .mentioned and at heating rates high enough for practical eiiiciency (eg. rates represented by the supply of electrical power at a density of 5 kilowatts or more per cubic foot of the material), have been distinctly unsuccessful, especially because of serious overheating in localized channels and fusing of the particles together along such channels or core-like paths. The problem is unusual, as distinguished from heating of refractory materials such as carbon or graphite, where there is also a negative temperature coefficient but incipient current channeling can be avoided or corrected by careful current control and especially by diffusion of heat to other areas. With the fusible alloy utilized Ifor the subhalide process, there is not only a very large negative temperature coellicient of resistance, with an extreme tendency to current channeling, but the fusing or localized agglomeration of the metallic granules along random or other paths, with ultimate freezing, i.e. solidication of the fused areas of contact, represents an essentially permanent impair-ment of the operation, which can neither be avoided nor corrected by reliance on mere diffusion of heat through the mass. Indeed diffusion promotes solidication. The properties of thermal conductivity of this material are relatively poor, and the formation, by surface fusion, of these agglomerated cores or masses is essentially intolerable and there is no practical way to get rid of them. As explained above, they clog the operation, an-dkmore Yso as they solidify; they tend to remain as preferential current paths, so that heating is non-uniform and efficiency of conversion, i.e. for treatment of an entire column, becomes poor; and they soon completely block the necessary movement of the column.

Thus inthe above cited U.S. Patent No. 2,937,082, procedures are described -for preheating the converter charge 3 by external application of heat, as by radiation from separate heater elements, or by allowing some reverse, decomposing reaction to occur in the produced monochloride gas as it traverses the incoming material, all with the object of effectively raising the fresh material to a high temperature, while avoiding the serious diiiicultiesexperienced in attempts to preheat by electrical resistance effect;

' It has now been found, however, that a relatively simple and eiiicient heating of the described particulate metallic material-:may be achieved with the instrumentality of electriccurrent passed through it, by -maintaining such material in suitable motion and in a body of fairly restricted cross-section, i,e. as a moving bed or column having certain dimensional characteristics and having a selected rate or character of movement, which is of substantially greater-rapidity or activity than utilized or encounteredin the stated conversion operation itself. Thus considered broadly, the present invention involves the discovery-that by appropriately moving the charge material, as down a vertical region or pipe which has a very substantially smaller cross-section than the columnar region inthe converter, so that the material advances at an'average rate which' is correspondingly greater than its slow travel through the converter and which is properly correlated with the rate of heating, such material can be effectively-preheated by passage of electric current through it Vin a direction coaxial with its path of advance, with an avoidance of serious current channeling and particularly with an avoidance of significant fusing of the particles or the consequent formation of elongated agglomerates or cores of the granular alloy.

In cooperation with such procedure, it is also found that passage of the material through one or more intermediate or terminal regions of larger cross-section, where some mutual redistribution of the particles can also occur, and resulting'or other equalization of temperature, sub-1 stantial further advantage is realized for permitting relatively high rates of heating in the narrower columnar path or paths. Accordingly, and preferably by utilization of all ofthe foregoing concepts, an effective preheating method or arrangement is'- realized, employing simple internal current vtlow for direct generation of heat by electrical resistance effect. j By thus heating the material in the described, mobile state, it is delivered-to the converter region without agglomeration and in a condition of uniform, high temperature, so-thatthereis no trouble in generating heat by current flow in such region to carry out the conversion reaction with trihalide gas. A specific feature of the'invention resides in the attainment of these results in heating the moving material at high p ower density appropriate for efficient operation of the monohalide refining process, i.e. a supplied power density of `at least 5 kilowatts per cubic foot, and indeed, for notable effectiveness (with the higher feed rates) `a substantially greater intensity, such as twice that value, or more.

; While the invention is not limited to any particular theory, it is believed that in any column or bed of granular particles of these alloys, the significant electrical resistance of the mass is mainly represented by the sum of (l) the contact resistance between particles, and (2) the resistance of the material in the region of highcurrent density (within the individual particles) close to the points ofcontact. Heat is chiefly developed, by current flow, at the principal points of resistance, and thus at and near the localities of contact between particles. While heat is eventually transferred back into the body of each particle, the electrica-l properties of the mass are primarily defined by the temperature at and adjacent the contact localities. In consequence theV situation of these localities is critical, and any overheating there quickly produces fusion, `under conditionswhich often are or become-unstable,leading to more localized overheating and melting, .while other Y 4 area-s, indeed other points of contact, may remain much cooler.

Indeed, beds of granular particles of these fusible metallic alloys show a negative temperature coeicient of resistivity even though the alloy in massive state may not: as the bed or column heats up, expansion causes an increase of contact pressure between particles reducing the resistance there, and as fusion begins and progresses, the local resistance decreases more and more, in the manner explained.

It has also now been found that the moving bed or mass of granular particles flowing down a pipe or the like, as in the new mode of procedure described above, does f not advance continuously (i.e., with a smooth, fully uniform velocity).but moves in a series of short pulses, Y

perhaps best described as jerks, each apparently occurring bythe transmission of a slight wave of motion up the column from the bottom thereof. At each of such jerks, it is believed that the points of contact between neighboring particles are disturbed and that to a large extent new points of contact are made. Hence new and different localities of heat generation are provided at each of the rather rapidly succeeding jerks Yor pulses of downward movement, i.e., when current is being passed through the bed forV the resistance heating function. In consequence by providing motion of this sort, with suiiicient frequency of pulses, as attained by selected speed .and conditions of travel of the material, the points of parl ticle contact are repeatedly broken and new points are established before there is an opportunity of melting to any significant extent, with the result that the heat distributes itself throughout the particles and the bed is essentially uniformly brought to the desired high temperature without appreciable local fusion. Y

For these results, the size of the bed and its mode of advance must be such that all particles `are frequently disturbed, to avoid development of one or more hot channels. These conditions, and likewise good current distribution, are facilitated or made possible by employing a column or other mass which has .a relatively small crosssectional area, a further important attribute of such dimensioning being that the length of heat diffusion paths through the body is then-short, as appropriate for material of relatively poorrthermal conductivity. In consequence there is fairly uniform heating throughout the whole cross-section.

A convenient, determining factor may therefore be deemed to reside in the pulse-type movement of the material, especially in the sense that with jerks of sufficient frequency, other necessary conditions can be readily determined to achieve the desired uniform heating by current flow. The jerks or pulses of movement are ascertainable by electricial measurement, it being found that with a nominally constant voltage across the supply electrodes, there are distinct and frequent current fluctuations indicating changes in resistance of the moving column. These are understood to be representative of the pulses of movement, whereby simple experimental data can be obtained in any case for ready determination of conditions (especially a sufficiently rapid frequency of jerks) for the desired result of preheating without fusion.

That is to'say, records of current fluctuation in a downwardly moving column (as just described) of this alloy material, when it is' subjected to current flow for electrical resistance heating, are interpreted as showing a gradual decrease in bed resistance in the temporarily sta-- ti-onary mass as the zones of contact warm up, then a p sharp increase in bed resistance as the mass jerks onward and the contacts are correspondingly re-arranged to form new, relatively cool, high resistance contact zones. Each suchcycle is observed as a gradual increase and sudden decrease in current, being understood to be caused by the above decrease and increase of resistance, and thus representing a cycle of rest and movement of the column. Present experience indicates that for useful feed rates of particulate alloy the frequency of these cycles is such that the average time between jerks, i.e., the average period from one pulse of movement to another may range from about to about 100 seconds. It will be understood that the duration of the intervals of rest is not ordinarily uniform, even for a given average feed rate. One convenient indication, however, of the condition of movement of a bed or column is the approximate maximum rest interval, which is defined below but which is exemplified, under certain tested circumstances of useful operation, by a value of about 30 seconds, or less, for rather high feed rates, and by values of about 60 to 100 seconds for considerably lower feed rates.

As explained, the dimensioning of the preheating column is to some extent related to the internal size of the converter, which in turn is designed to afford efficient performance of the reaction to produce monochloride from the alloy. Thus in the converting region, the dimensional proportions and the rate of alloy travel should be such that the alloy remains in the traversing ow of gaseous aluminum trichloride for a sufficient time to convert a suitably high percentage of the aluminum content. For such results the dimensions of the region are further governer by the requirements: that sufficient heat be generated by passage of electric current, to serve the endothermic reaction, and that the gas traverse the region in sufficient contact with the alloy, yet without such pressure drop as would unduly limit the rate of gas passage that would be practical.

Under conditions thus selected to afford good eiciency of conversion, as for example to achieve extraction of 90% yor more of the aluminum content in alloys containing, say from 50% to 60% Al, it is found that the actual time of passage of alloy particles through the converting region, e.g. downward, is generally about one day (24 hours) and certainly at least one half day (12 hours). Correspondingly, the cross-section of the slowly traveling bed or column, in the converter, is relatively large, in order to obtain commercially signicant production of pure alminum in the system. Thus in general an upright cylindrical converter should have an internal diameter of `at least about 5 feet, and preferably 7 feet Ior more, the `cross-sectional area of this or other configuration varying upward from a minimum of about 20 square feet or a preferred minimum of about 40 square feet.

As also indicated above, it has been found that in utilizing an internally electrically heated body of particulate alloy in such a converter, for example with the arrangements and procedure just described as app-ropriate to function efficiently for conversion of aluminum to monohalide, the alloy must be preheated to a high value, e.g. 700 C. or above, and preferably near the conversion temperature (depending on the extent of other modes of preheating that may be inherently or specifically embodied in the converter), before feeding it to the converter, or difficulties of fusion between particles and of severe current channeling may arise.

In carrying out the present invention for preheating of the alloy feed, as for converter operation of the sort described above, the preheating column should have a cross-section substantially smaller than that of the converter, indeed usually no greater in area than about half of the cross-sectional area of the converting region, and preferably considerably smaller, as in the case of a preheating column with a diameter of one-half to one-third that of the converter, or less. Considered independently, a preheating column of particulate crude alloy, through which current is passed substantially parallel to the axis, i.e. lengthwise of the path of travel, should in most cases have a diameter selected in the range from about five times (or better at least six times) the average particle diameter to about seven times such average diameter plus six inches, where the average particle diameter is in the range of four inches or less, preferably in a range from one-half inch to four inches and most advantageously from one inch to three inches. A particularly useful size of preheating column is one having a diameter of about six to about eight times the average particle diameter.

Thus, for example, a -suitable preheat column diameter, to feed a converter having a diameter of five feet, and utilizing a granular alloy having a nominal or average particle diameter of two inches (with individual particles ranging from one-half inch to two and one-half inches), is found to lie within a range of ten inches to twenty inches, and preferably fourteen to sixteen inches. In such circumstances, the converter is expected to handle an alloy feed in the range of 2000 to 6000 pounds per hour, or somewhat more, and effective preheating is then attained (using coaxial current flow in the preheating column) without appreciable fusion together of the feed particles. That is to say, at these feed rates in a preheat column as described, the frequency of pulses or jerks of downward movement of the material, by gravity, is adequate for the desired breaking of electrical contacts and re-orienting of the particles, and indeed lies Within the limits in the above examples, whether determined by the average or by the approximate maximum rest interval.

The invention will be further understood in connection with the accompanying drawings, wherein:

FIG. 1 is an essentially diagrammatic view, in the nature of a vertical section, of a converter and a feeding and preheating system therefor, embodying the invention as an example of the apparatus thereof;

FIG. 2 is an enlarged view in vertical section of the preheating and certain associated portions of the system as shown in FIG. 1, with the structure illustrated in simplified form; and

FIGS. 3 and 4 are respectively sections on lines 3-3 and 4 4 of FIG. 2.

As shown in FIG. l, the converter 10, which may be generally similar to the structures shown in the abovecited patent, is an upright cylindrical chamber or furnace having upper electrodes 12, 12', of carbon, graphite Aor the like extending through the walls at two or more localities, and the like lower electrodes 13, 13' whereby electric current, e.g. alternating current from a suitable source 14, may be passed between the electrodes 12, 12' and the electrodes 13, 13', through the contained mass of charge material 15. The converter 10, and indeed all of the other structures hereinafter described which are intended to enclose the alloy material in a heated condition or while it is being heated, may be constructed with an external, sealed, steel shell (not shown for the converter in FIG. 1 but depicted for other vessels at 16, 17 in FIG. 2), having a heavy internal lining 18, 18 (FIGS. l and 2) of suitable refractory material such as dense highly pure alumina or other substance which is inert to the gases involved in the process `and has suitable electrical insulating character so as not to bypass any appreciable current.

The Zone defined between the upper and lower electrodes 12, 12 and 13, 13 represents the reaction region and is kept entirely filled with the granular alloy at all times, i.e. to a level above the upper electrodes. Preheated aluminum trichloride gas enters through pipe 20 and is released within the converter by suitable distribution means 21, i.e. below the lowermost electrodes, and then flows upwardly through the bed of alloy, wherein by reason of heat energy from the current, the AlCl3 converts aluminum of the alloy to gaseous monochloride. Near the top of the converter, a refractory lined discharge conduit 22 carries the product gas, consisting of aluminum monochloride (AlCl) and unreacted aluminum trichloride, to the decomposer (not shown) where under the influence of appropriate cooling means, to extract heat, the monochloride decomposes to yield pure aluminum and aluminum trichloride. Fresh alloy feed enters the converter at the top, i.e. through an inclined feed conduit 24, while the aluminum-depleted residue of granular solid leaves the bottom of the converter, as through a discharge conduit 25, advance of the spent alloy out of the converter being aided by suitable mechanical means, such as a conventional discharge cone 26, carried on and rotated by a vertical shaft 27. t

The feed system, which may be a long upright series of devices above the converter 10, includes a supply hopper 30, receiving the granular alloy and guiding it into the column structure shown below the hopper. This column structure may include suitable lock or valve means whereby the alloy may advance downwardly, in successive increments, to a lower, enclosed feed hopper 32, without appreciable leakage of reactive chloride and monochloride gases and without appreciable access of air to the structures below. Such lock or valve system may consist of a vertically spaced pair of valves 33, 34 in the feed column assembly 35, with an intermediate hopper 36 between them, the hopper 36 being arranged with a valved inlet pipe 37 for introduction of inert gas at desired times. Although the valves 33, 34 may be of any appropriate character for interrupting the downward path of alloy and for essentially closing the path against liow of gas except when opened for alloy passage, they are shown as simple star valves, being known devices for this type of function.

Below the lowermost feed hopper 32, the preheating section comprises an upper preheat column 40, a lower preheat column 41, and between them a mixing chamber or hopper 42 of substantially wider cross-section. Having in mind that the column 40, chamber 42 and column 41 are kept filled with the downwardly traveling, granular alloy, means are provided below the column 41 for advancing or aiding the advance of the material into the inlet conduit 24 of the converter, such means 4being of appropriate sort, such as a table, cone, screw or other feeding device, the illustrated equipment specifically embodying a rotating circular table 43 for this purpose, within a lower feed chamber 44.

Referring also to FIGS. 2 and 3, it will be seen that each of the preheating sections 40, 41, is an uprightV columnar chamber, refractory lined, having an internal, vertically elongated cylindrical configuration, and provided with electrode means for passage of current through the moving material. Thus the upper column 40 has vertically spaced sets of graphite, carbon or other suitable electrodes 45, 45 and 46, 46', which project to or into the enclosed feed path. The lower column 41 has similar upper and lower sets of electrodes 47, 47 and 48, 48.

The mixing chamber 42 through which the material passes from the upper preheat column 40 to the lower preheat column 41, comprises an inner cylindrical section 50, from which a cone-shaped region 51 extends or tapers downwardly to the column 41, while an upper, flaring or cone-shaped region 52 extends from the lower end of the column 40 to the top of section 50,

In the discharge chamber 44, the horizontal, circular, rotating table feeder 43 is disposed, being carried by a vertical shaft 53 below it, suitably driven -by means not shown. A stationary scraping or pushing blade 54 may be mounted above the table at an outer part of its upper surface, e.g. on the side adjacent the conduit 24, so that as the table turns the material is pushed or aided in falling from the table into the latter conduit. Coacting guide means may be carried by the lower side of the table 43, e.g. a chain structure 55 which aids in sweeping material into the duct 24.

Electric current for the preheating columns is furnished from a suitable source, for example an alternating current source 56. As shown in FIG. 1, the upper pair of electrode sets 45, 45 and 46, 46 is supplied with current through a transformer 58, while the lower pair of electrode sets 47, 47 and 48, 48 is supplied through a like, similarly connected transformer 59. The heating current circuit of the electrodes of each column includes an inductive reactance coil, e.g. the coil 60 in the upper circuit and the coil 61 in the lower, which substantially stabilizes the operation. Without these coils, there may be large fluctuations in power input, that may create some difficulty, although not insuperable, in control of the operation.

Electrical balancing means may also be employed in order to distribute the current between the electrodes .and thereby avoid any preferential passage of current through one electrode or the other with corresponding tendency to non-uniformity of current flow through the column, such balancing coils being shown at 62, 62', `63 and 63. As shown in the illustrated system which includes two rather than Imore electrodes in each set,'the two electrodes 45, 45 are respectively conncted to the ends of the coil 672 which has its mid-point connected to one Side of the secondary of the transformer 58, and in this instance through the stabilizing coil 60. The remaining balancing coils 62', 63 and 63 are similarly connected to the remaining pairs of electrodes, respectively 4646, 4747 and 48-48. It may be explained that the transformers 58, 59 are designed or adjusted (as by tapped or other variable windings, e.g. a tapped primary winding, as shown) to afford desired voltages respectively for the upper and lower heating columns and also for appropriate regulation of power inputs to these columns.

Although alternating current is of special advantage for energizing the several heating circuits shown, direct current (under suitable controls) can be used if available and desired.

Inasmuch as the resistivity of the feed alloy in the columns 40, 41 decreases as its temperature rises, it is usually necessary, especially for good electrical eiiiciency, to apply different voltages to the respective column sections. Thus the voltage between the electrodes of column 40 is conveniently substantially higher than that applied across the electrodes of column 41. The resistance of the body of material in column `40, considered as a unitary mass, is correspondingly higher than that of the hotter body in column 41, making the above difference of voltages requisite or at least desirable to achieve high power input and maximum heating in both columns.

The following is given by way of example of a suitable two-sta-ge heater such as shown in the drawings, for supply of granular alloy to a cylindrical converter 10 having an internal diameter of 5 feet and a vertical'distance between upper and lower electrodes 12, 12 and 13, 13 of about 25 feet. Each of the columns 40, 41, in this example, has an internal diameter of 14 inches Iand a vertical distance between upper and lower electrodes (center to center) of 93 inches. The intermediate mixing chamber 42 has a diameter at its maximum width region 50 of 39 inches, and a total capacity, between columns, of about 27.5 cubic feet.

In one series of operations of such a heater, the alloy entered the top of the first-stage heater, column 40, at approximately room temperature and was there heated to 400-500 C. under an Iapplied Voltage of 70 to 140 volts. In the second-stage heater, column 41, the 4alloy was carried from the above terminal temperature of the first stage, to a substantially higher value, i.e.` 700 C. and above, by current at a voltage of 40 to 70 volts. These results were attained with feed rates ranging up to 6500 pounds per hour, e.g. upwards of 2500 pounds per hour. At an actual start-up with cold alloy throughout a given column, considerably higher voltages are temporarily re-V quired, e.g. of the order of to 300 volts; the ranges given above .are for running operation after the working temperature gradient along the column has been reached.

It will be understood that the actual charging and mechanical manipulation of the .apparatus is relatively simple. At the outset the valves 33, 34 are closed and hoppers 36, 32 are considered empty, the top hopper 30 being full of carbothermic alloy, e.g. irregular particles or fragments as resulting from breaking or crushing largerV pieces, and having particle size characteristics as elsewhere herein explained. The top valve 33 is first opened and simultaneously a stream of inert gas is supplied through the pipe 37, venting through the top valve and hopper 30 as alloy descends into the hopper 36. The valve 33 is then closed and the lower valve 34 opened allowing alloy to descend into the third hopper, and thence to the lower shaft system, i.e. the preheating column 40, chamber 42 and column 41. By successive like operations the hopper 32 is lreiilled as necessary so that a continuous supply of feed is maintained for the system.

With the feeder 43, alloy is continuously removed from the preheat column and dropped into the discharge chute 24. The speed of rotation of the table 43 bears a relationship to the rate of removal of alloy from the column and can be employed, within limits, to govern the general or average rate of downward travel of alloy under the force of gravity, i.e. the through-put. This rate, of course, should be selected to agree with the desired through-put in the converter 10, the latter being chosen for desired results and being adjusted or controlled, not only in accordance with the gravity head of the material in the converter but also by varying the speed of the discharge device, e.g. cone 26, utilized for aid in removal of spent charge.

As specific examples of operation of `a preheating system having the above dimensions and relations, the following table sets forth typical electrical characteristics for various feed rates of an aluminum-containing carbothermic alloy, expressed in pounds per hour. The composition of the alloy was: aluminum, 5053%; iron, 28- 3l%; carbon, 3.5-4.5 silicon 8.5-9.5 titanium, 3.3- 3.6%. In the first .and second stages respectively (e.g. as in columns 40 and 41) the moving alloy was raised from room temperature to .an average of about 500 C. and from the latter value to an average of about 700 C. The power supplied in the respective stages is given as kilowatts required, the average power density (kw. per cubic foot) in each stage (having a cross-section of about one square foot) being then equal to the stated value divided by the height of the column, in feet, between electrodes. As indicated in the table, each stated value of nequired power included about 20 kilowatts of heat loss. The table also sets forth the total resistance and the bulk resistivity (averaged `over the column) of the body of particulate lalloy in each stage, for the stated conditions of feed rate and current ow.

m-aterial can be described as non-ohmic: it decreases with increase in temperature and with increasing current density. As employed herein, the resistan-ce of a column means the bulk resistance of the entire mass of particles in the column, as compacted by their own weight; resistivity of a column or bed similarly means the average bulk resistivity of the mass of particles; and each reference herein to a temperature coefficient of resistivity is characterized explicitly or by context, to have a selected one of two meansings, viz (a) a property of the column or bed in bulk, or (b) a property of the solid material itself, e.g. of massive alloy, as in the body of a single particle.

Other tests have demonstrated that this alloy, in particulate form, and indeed in the particle size ranges mentioned above, may be readily heated by operations of the type described and with apparatus as shown in the drawing, to bulk temperatures of more than l000 C. without diculty from fusing. Indeed experience indicates that up to temperatures near the softening point of the alloy (for example, about 1300J C. in the alloys treated above), the higher the temperature, i.e. temperature reached at the outlet of the preheater, the better the system performs.

In these further tests, the columns employed had an inside diameter of 14 inches and a length of 7 feet between electrodes. The carbothermic alloy used had a particle size range between about one-fourth inch and about two and one-half inches, being regarded as a nominal or Iaverage particle size of about one and one-half to two inches. Feed rates of 1500 to 6000 Apounds per hour were utilized, being equivalent to about 135 to 550 cubic feet per hour of the ybulk material as it descended under gravity in the columns.

As described above, chart records of the current through a preheating column of the sort here shown (e.g. column 40 or column 41) show a continuing sequence of current fluctuations, with sharp decreases and gradual rises. As .also previously stated, these sharp decreases, regarded as major current iiuctuations, are understood to correspond to the pulses of downward movement of the alloy, when the particles shift and are re-oriented with fresh, relatively cooler points of contact and thus relatively hi-gher regions of electrical resistance. The following table, derived from counts of these major current fluctuations over a considerable number of r-uns of successful opera- TABLE I KW required Typical Feed Rate, (includes lb./hr. 20 kw. heat l 105s Current, Volts Resistance Resistivity,

amps (milliohrns) milliohm-cm.

First Stage 500 C. out 1,000 80 1, 300 62 48 200 2, 000 140 000 7() 35 147 Second Stage 700 C. out 1, 000 42 1,150 36. 5 32 133 2,000 67 1, 600 42 26 110 tion, summarizes the time values of the intervals between fluctuations, for various feed `rates of alloy:

TABLE II Time to include stated Feed rate of -21/224-1/4 percentage of fluctuations earbothermic alloy 50% (see.) 95% (sec.) 99% (sec.)

2,000 pounds per hour 26 75 93 3,000 pounds per hour 18 50 62 4,000 pounds per hour 14 37 46 6,000 pounds per hour 9 25 30 It will ybe understood that the percentages at the head of each of the columns of time values (in seconds) refer to a measure of averaging or of numerically characterizing the rest periods: e.g. at 2000 pounds per hour, 50% of the intervals .between fluctations were 26 seconds or less, and 99% of such intervals were 93 seconds or less. Hence it will be seen, at least to a rough approximation, the greater or greatest intervals between downward jerks or pulses were about 75 to 100 seconds at the lowest of the feed rates and about 25 to 30 seconds at the highest. Indeed the term approximate maximum interval between pulsations is adopted herein as a convenient measure of the pulse frequency, and signifies that period Aof time within which 99% of the uctuations fall, i.e. values as exemplified by the last column in Table Il. Experience indicates that this is a reliable useful Way of evaluating the pulse-type movement. While the permissible rest interval (e.g. taken as the approximate maximum) is inu# enced somewhat by the temperature and size distribution of the alloy, Table II nevertheless indicates approxmiately a rather wide range of pulse times vfound useful for successful operation. It is conceived that somewhat longer times can lbe employed, as with somewhat longer columns.

It may be noted that in the examples above, employing vfeed rates as in Table Il and higher, in columns dimenioned as stated, the downward velocity of particles .averaged from 3.4 to 10.2 inches per minute.

As indicated, the procedure and apparatus herein described are concerned with beds of fusible conductive material, e.g. solid metallic material in divided, i.e. particulate form (usually having particle diameters from about one-fourth inch to about five inches, and an average particle -diameter selected in the range from about one inch to about four inches), which have a negative coefficient of resistivity over a range of elevated temperatures (e.g. at least several lhundred degrees C.), it being desired -to heat the material over a substantial part of such range.

The invention is particularly concerned with alloyed r otherwise inseparablymixed or aggregated metals, including a substantial but not necessarily major content of aluminum, being material, such as the described carboA thermic alloy, utilized for separation of aluminum therefrom in the monohalide refining process. These are alloys which remain essentially in solid state at the temperatures, i.e. upwards of 1000" C., employed for conversion. A negative temperature coeicient of resistivity, over a range approaching conversion temperatures, is charactertistic of massive bodies of most of such alloys or crude metals, e.g.V as given below, and of granular beds or columns of all of them, i.e. including compositions having relatively high aluminum content, single .pieces of which may show little change of resistance with temperature.

In the examplesv given herein, the carbothermic alloys employed, in granular, solid form, generally had compositions within the following range (excluding slight or trace amounts of other metals):

Beds of granular alloys of this character have a resistivity at C. within the range of about 50 to 5000 ohmcentimeters. The resisitivity drops yrapidly with increasing temperature to a value of about 0.1 ohm-centimeter at 1000" to 1100" C., and thereafter decreases only very slowly (indeed relatively insignificantly) to a value notV lower than about 0.05 at 1400" to 1500" C.

It appears generally adequate, for proper operation of the converter 10, to supply the alloycharge at about 1'000" C., even though the operating range for conver- 12 sion may be of the order of 1200" to 1400" C; Indeed itis possible in *many cases, especially with a lower rather than higher through-put of material, to supply the converter with charge at temperatures as low as 700 to 800 C., although in general, best avoidance of problems of non-uniform heating or local fusion, in the converter, appears to result with somewhat `higher temperature of the feed.

The electrical preheating operations can also .be employed in conjunction with other modes of preheating, for instance in that the function of the column or columns above the converter may be limited to reaching some temperature well below 1000" C., and one or moreother preheating practices, eg., as disclosed in the aforesaid U.S. Patent No. 2,937,082 and performed in upper zones of the converter vessel itself, may be utilized to get the granular charge material into a highly heated state, even to l200 C. or above, as it reaches the uppermost electrodes 12, 12' of the converter.

As will now be seen, the present invention affords an effective and reliable mode of heating the fusible metallic material, utilizing the passage ofV electric current through suchmaterial, and particularly being able to take advantage of the efficient manner of operation wherein the current travels along ya path or paths coaxial with, i.e. the same as or parallel to, the path of movement of the material, whether the latter is directly vertical, or in some other direction, e.g. downward at an incline. Indeed notably in an upright column, this mode of current supply has special significance in aid of avoiding fused linkages of particles. Y

Although not essential in all cases, the equalization or mixing chamber 42 is of material consequence in the process, alfording not only some further, considerable shifting of contacts among the particles, but also providing equalization of temperature, both by distribution of the more highly heated granules and also by increased opportunity for conduction of heat throughout the mass. The time of retention of a given particle, i.e. its time of passage through the chamber, depends obviously on the rate of feed, Ait being found that as a very rough approximation, a typical and suitable time of retention for particles in this chamber 42 was about three times the interval of retention between electrodes in each of ,the columns 40 above and 41 below. As will be appreciated, some further equalization, as well as redistribution, occurs as the heated material traverses the chamber 44, and is advanced by the table feeder 43 down through the chute 24.

The number and dimensions of the preheat columns, and likewise of intermediate equalization chambers (which may in some cases be simple columnar connections of no greater diameter, between the electrically energized zones), can be selected to suit any desiredcircumstances. Thus in some cases a single column may suflice, and in others two or more columns of equal size, preferably separated by an intermediate chamber, may be employed. In general, it is preferred to use columns of as small diameter as will accommodate the desired feed rate and as will not impede the deired flow of the material. As stated, any desired number of stages can be used, with any desired temperature rise in each.

Although it is conceived that the process may be carried out with other means, even such equipment as a very slowly rocking or rotating, cylindrical rotary kiln, suitably inclined and refractory-lined and having embedded ring or other electrodes for passing current through the contained moving mass, or indeed with other suitable apparatus involving a bed of particles having appreciable, even though slight, freedom of movement relative to Veach other in consequence of advance of the bed or major portions of it, the use of upright columns (or a plurality of column systems in parallel) through which the bed of particulate material travels downward under the influence of gravity, is especially effective in achieving the desired character of motion .and reorientation of contacts (by virtue of such freedom of movement) while maintaining sufficient pressure between particles to assure a large multiplicity of current-carrying contacts `at all times. In general, these and similar modes of handling the material (while passing current through it) inherently tend to be characterized by the defined pulsating movement as the bed is caused to advance downward or otherwise along the containing vessel. As will be understood, violent agitation of the material is undesirable and should usually be avoided, e.g. in Athat the result of impact and mutual abrasion between the lumps or granules of carbothermic alloy very quickly builds up an undue quantity of fines, i.e. extremely fine particles, which if excessive tend to clog the mass against desired passage of aluminum trichloride gas therethrough in the converter.

It will now be understood that the factors influencing the rate and extent of heating achieved with the presently described yprocedures include the melting point, heat conductivity and electrical conductivity of the material, as well as its negative temperature coefficient of electrical resistivity, and of course, the bulk temperature to which it must be heated. It should be apparent to those skilled in the art that these factors can be readily taken into account for application of the process to any desired solid, particulate material of the character described, i.e. including other metallic aluminum-containing materials as well as the electrothermal or carbothermic alloys specifically mentioned.

A major but easily determined factor is the speed of movement of the material, epecially the rate at which particle-to-particle contacts are broken and the particles re-oriented, i.e. the frequency of pulsations or jerks in the downward travel of the bed. With the heating rate appropriately selected to suit the characteristics of the material (as explained), the desired results are easily achieved by designing the system and operation to have a suitable frequency of movement of the particulate mass. The heating rate can be taken, for a given material, as power density, i.e. supplied power (including that required to account for heat losses) per cubic foot, averaged over the height of the column. Using the approximate maximum interval between pulsations as the measure of frequency of movement of the material, it will be appreciated that the permissible value of such maximum interval decreases with increase of temperature maintained at the outlet of the column, and especially decreases with increasing power density in the column; for instance, tests have indicated, with 'operations as in above examples, that as the power density rises to very high values (eg. of the order of 40 kw./cu. ft. and upwards) the interval should be substantially less than the largest value in Table II, this being in effect apparent from Tables I and II. A number of values of the approximate maximum pulse interval found suitable under a variety of conditions are stated herein, but in any event, :appropriate characteristics of bed travel (for given circumstances) can be readily determined by simple test in the light of the principles set forth, bearing in mind that the frequency of pulses of movement is sufficient if it does not permit appreciable mutual adhesion of the particles by fusion. In some cases the approximate maximum interval between pulses can be relatively long, e.g. 3 minutes or more, especially where lower heating rates (such as power densities down to 5 kw./ cu. ft.) are employed and longer columns or more equaflizing zones are utilized to take best advantage of such rate.

It is to be understood that the invention is not limited to the procedure and apparatus herein described but may be carried out in other ways without departure from its spirit.

We claim:

1. A method of heating a continuing feed of a mass which is composed of particulate, solid metallic material and which has a negative temperature coefficient of resistivity over a range of elevated temperatures, for raising the ltemperature of said particulate material to a predetermined value at least substantially higher than the lower limit of said range, from a value substantially below said predetermined value, comprising advancing said mass of metallic particles along a predetermined downward path while passing electric current through said mass to generate heat therein, said advance of the mass of particles including imparting pulsations of movement, through the mass, which produce mutual displacement of individual particles, and said pulsations being effected with sufficient frequency to prevent substantial mutual adhesion of particles in the mass by vfusion under the heating influence of the current, said mass being advanced by moving it under gravity along said downward path in a confined, columnar region, said downward movement being effectuated with the aforesaid pulsations by discharging the material of the mass at a locality below said region at a rate selected to impart said pulsations with the aforesaid sufficient frequency.

2. A method as defined in claim 1, wherein said current is passed along paths parallel to said path of movement.

3. A method as defined in claim 2, wherein the material is an aluminum alloy in particles having size characteristics selected within a range of about 1i-inch to about 5 inches, said mass being moved downward in said region at a rate of at least 2000 pounds per hour and through a distance greater than several feet, while said current is passed in said region through a vertical extent `of said material of at least several feet, said current being applied at a selected power density of at least 5 kilowatts per cubic foot of the mass, and said mass being advanced by pulsations of movement having an approximate maximum interval between them sufficiently less than three minutes to prevent substantial fusion of particles to each other under the heating effect of the selected power density.

4. In a process for separation of aluminum by subhalide -distillation at 'a predetermined elevated temperature from masses which are composed of particulate, solid metallic material containing aluminum and other metal and which have a negative temperature coefficient of resistivity over a range of elevated temperatures below said predetermined elevated temperature, wherein said material is treated for conversion of aluminum therein to 'aluminum subhalide, by advancing the material through an enclosed zone while heat energy is there supplied to the material and while gaseous h-alide is there passed in contact with the material for conversion reaction with the aluminum to produce aluminum subhalide in gaseous form: the method of preheating said particulate metallic material while advancing the same to said conversion zone, which includes advancing said particulate material as a mass thereof along a predetermined path while passing electric current through said last-mentioned mass to generate heat therein to raise the temperature of said mass to a value substantially above the lower limit of the aforesaid range of elevated temperatures, said advancing of the mass of particles including imparting pulsations of movement through the mass which produce mutual displacement of individual particles, and said pulsations being effected with sufficient frequency to prevent substantial fusion of particles to each other under the heating influence of the current.

5. In a process for separation of aluminum by subhalide distillation at a predetermined elevated temperature from masses which are composed of particulate, solid metallic material containing aluminum and other metal and which have a negative temperature coefficient of resistivity over a range of elevated temperatures below said predetermined elevated temperature, wherein said material is treated for conversion of aluminum therein to aluminum subhalide, by advancing the material through a zone of predetermined cross-sectional area and at a predetermined rate while it is heated by passing electric current therethrough `and while gaseous halide is passed in Contact with the material in said zone for conversion reaction with the aluminum to produce aluminum subhalide in gaseous form: the method of preheating said particulate metallic material while advancing the same to said conversion zone, which includes continuously moving a mass of the particulate material through an enclosed Y zone, for `said preheating, having a substantially smaller cross-sectional area than the converting region, said movement of the material being effected at a substantially faster average speed of the particles than the advance of :material through the converting region, while passing I electric current through the moving material in said preiheating zone to supply heat energy thereto, the said faster fspeed of movement of the material and the said smaller 'cross-sectional area of said preheating zone cooperating to provide sufficient mutual displacement of particles of the material to prevent substantial mutual adhesion of said particles by fusion, while heating the material by said last-mentioned current to an elevated temperature substantially above the lower limit of the aforesaid range of elevated temperatures.

6. A method as defined in claim 5, wherein moving the mass of the material through the preheating zone comprises moving said mass downward by pulses under gravity through said Zone of columnar configuration, while passing said last-mentioned electric current through paths parallel tothe axis of the column between upper and lower electrodes exposed to said mass, said downward gravity movement and said cross-sectional area of the preheating zone being respectively controlled and selected for displacing the mass by said pulses of movement having a frequency to disturb the particles relative to each other to provide the aforesaid sufiicient mutual displacement `of the particles.

7. A process as defined in claim 5, in which the preheating method includes moving said last-mentioned mass of particulate material from the aforesaid enclosed preheating zone through an enclosed region, for equalizing temperature throughout the material, and then through a second enclosed zone, for further preheating, said movement of thematerial through said second preheating zone being effected at a substantially faster average speed lof the particles than the advance of material through the converting region, while passing electric current through the moving material in said second preheating Zone to supply heat energy thereto, the said faster speed of movement of the material and the said smaller cross-sectional area of said second preheating zone cooperating to provide sufficient mutual displacement of particles of the material to prevent substantial mutual adhesion of said particles by, fusion while heating the material by said current in the second preheating zone to a temperature :substantially higher than the temperature to which the material is heated in the first preheating zone.

8. A method as defined in claim 7, wherein the advance of the material through each of the preheating :Zones comprises moving the mass downward by pulses under gravity through such preheating zone, of columnar configuration, while passing electric current through paths 4parallel to the column between upper and lower electrodes exposed to the mass in such preheating zone, said movement of the material and the dimensions of the column, `of each preheating zone, being respectively controlled and selected for displacing the mass of said pulses of movement having a frequency to disturb the particles relative to each other sufliciently often for preventing appreciable fusion of the particles to each other.

9. A method as defined in claim 8, wherein the Vparticles of material have size characteristics within a range of about 1/2-inch to aboutl 4 inches and an average size selected in the range of 1 to 3 inches, said material having a negative temperature coefficient of resistivity through a temperature range from about 400 C. to at least about 1000" C., each ofsaid columnar preheating zQIfleS having a vertical distance of at least several feet between electrodes through which distance the mass is moved downwardly, said heating by the current in each said preheating zone being effected at a selected power density, of at least about 5 kilowatts per cubic foot,'to raise the temperature of the material in the first zone by at least 200 C. to a value in the aforesaid last-mentioned temperature range and to raise the temperature of the material in the second zone by at least 200 C. more to a correspondingly higher value in said last-mentioned range, said method including controlling the descent of material through the zones so that the mass is advanced in each Zone by pulsations having an approximate maximum interval between them sufficiently less than three minutes to provide the aforesaid sufficient disturbance of the particles to prevent appreciable particle fusion under the selected power density in each zone.

10. A method as defined in claim 8, which includes displacing the material transversely during its advance between said preheating Zones by moving it through an enclosed region, as aforesaid, having a cross-sectional area substantially larger than both of said preheating zones, to enhance equalization of temperature and uniformity of heating of the material by re-distributing the particles among Veach other for modifying their mutual contacts while slowing their advance between Zones for enhanced diffusion ofl heat.

11. A method as defined in claim 10, which includes continuously displacing the particles of material from a locality below the second preheating zone to advance the material into the conversion zone, for effectuating the downward movement of the mass through the first preheating zone, the` intermediate enclosed region and the second preheating zone by pulses of movement, said displacement of the particles being effected at a rate selected to provide the aforesaid frequency of pulses for said sufficient disturbance of the particles in each of Vsaid preheating zones.

12. In a process for separation of aluminum by subhalide distillation at a predetermined elevated temperatureV of at least -about l000 C. from particulate, solid metallic material which contains aluminum and other metal andV determined cross-sectional area while said mass is heated by passing electric current therethrough and while gaseous aluminum trichloride is passed in contact with the material in said zone for conversion reaction with the aluminum to produce aluminum monochloride in gaseous form: the method of preheating'said particulate metallic material while advancing the same to said conversion zone, which includes moving the material, as a mass of l particles in contact with each other and with freedom of individual displacement during said movement, through an enclosed zone having a cross-sectional area smaller than one-third of said area of the conversion zone, for said preheating, while passing electric current through the material in said preheating zone along Vpaths substantially parallel to the path of advance of the material through said preheating zone, to supply heat energy to said lastmentioned mass, said movement of said last-mentioned mass of particles including imparting pulsations of movement throughout the mass which produce mutual displacement of the individual particles, while limiting said mutual displacement to prevent appreciable disintegration of the particles, and said pulsations being effected at a rate such that the approximate maximum interval between pulsations is sufficiently short to prevent substantial fusion of particles to each other, while heating the last-mentioned mass by said last-mentioned current to an elevated temperature substantially above the lower limit of the aforesaid range of elevated temperatures.

13. A method as defined in claim 12, wherein the advance of the material through the preheating zone comprises advancing said last-mentioned mass of the material downward by gravity through said preheating zone while controlling discharge of said material below said preheating zone into the conversion zone, to maintain the aforesaid rate of said pulsations.

14. A method as defined in claim 13, wherein the particles of material have size characteristics selected within a range of about 1t-inch to about 5 inches and an average size selected in the range of l to 4 inches, and wherein the downward movement of the particles effects pulsations of movement having a frequency such that the approximate maximum interval between pulsations is not more than about two minutes.

15. In a process for separation of aluminum by subhalide distillation at a predetermined temperature of at least about 1000 C. from particulate, solid metallic material which has particle size characteristics selected within a range of about 1i-inch to about 5 inches, and which contains aluminum and other metal, and which in the form of a columnar mass exhibits a negative temperature coefficient of resistivity through a temperature range that extends `over at least about several hundred degrees from a lower value that is substantially above room temperature, wherein said material is treated for converting aluminum therein to aluminum monochloride, by advancing a colu-mnar mass of the material downward by gravity through a zone of predetermined cross-sectional area and at a rate of at least about 2000 pounds per hour while heat is supplied by passing electric current through said last-mentioned mass, thereby maintaining the material at said first-mentioned predetermined temperature, and while gaseous aluminum trichloride is passed in contact with the material in said zone for reaction to produce aluminum monochloride in gaseous form: the method of preheating said particulate metallic material while advancing the same to said converting zone, which includes advancing a columnar body of the material downward by gravity through a zone of upright columnar configuration, which has a cross-sectional area smaller than one-third of said area of the converting zone, while passing electric current through the material along paths parallel to the column between vertically spaced electrodes exposed to said body in said columnar zone, the particles of said body being maintained in contact with each other and in condition yof limited freedom of individual displacement during said downward advance, and said downward travel of the material by gravity in said columnar zone being effected at a selected rate which provides a downward average particle speed substantially faster than in the converting zone but which is limited to prevent appreciable disintegration of the particles, for advancing said body by pulses of movement that produce mutual displacement of the individual particles with such frequency that the approximate maximum interval between pulses is sufficiently short to prevent substantial mutual adhesion of particles by fusion, while heating said body by the electric current at la power density in said body of at least 5 kilowatts per cubic foot, to raise its temperature through at least a part of the aforesaid range.

16. In apparatus for producing purified aluminum by subhalide distillation fro-rn particulate metallic aluminumcontaining material, in combination, a converter comprising an upright columnar vessel adapted to receive a continuing downwardly traveling mass of said material, having vertically spaced electrode means for passage of current through the material in the vessel for supply of heat therein, said vessel having gas inlet and outlet means at respectively opposite ends thereof, and having a predetermined internal cross-sectional area, and means for preheating the particulate material for supply into said converter, comprising upright columnar structure above the converter vessel, to receive a continuing mass of said particulate material for downward advance, vertically spaced electrode means in said columnar structure for passage of current through the material to preheat the same, said columnar structure having an internal cross'- sectional area substantially smaller than the aforesaid cross-sectional area of the converter vessel, to provide substantially more rapid downward advance of the pafticulate mass in the columnar structure than in the converter, and means providing a path for said particulate material between said columnar structure and the converter vessel. y

17. Apparatus as defined in claim 16, wherein the upright columnar structure comprises va plurality of vertically successive columns each adapted to be filled with the descending particulate material and each having vertically spaced electrodes for passage of current through the material, each of said columns having an internal cross-sectional area substantially smaller than said area of the converter vessel, Iand enclosed means intermediate and communicating with the columns, providing enclosed space for travel of material between successive columns, having a cross-sectional area substantially wider than the columns adjoining said space, for slower travel of material than in the columns, to equalize the temperature of particles across the mass.

18. In apparatus for producing purified aluminum by subhalide distillation from particulate metallic aluminumcontaining material, in combination, a converter comprising an upright refractory-lined columnar vessel adapted to receive a continuing downwardly traveling mass of said material, having vertically spaced electrode means for passage of current through the material in the vessel for supply of heat therein, said vessel having gas inlet means at a lower part thereof and gas outlet means at an upper part thereof, and said vessel having an internal crosssectional area of at least about 20 square feet, and means for preheating the particulate material for supply into said converter, comprising upright refractory-lined columnar structure above the -converter vessel, to receive a continuing mass of said particulate material for downward gravitation-al advance, vertically spaced electrode means in said columnar structure for passage of current through the material to preheat the same, said columnar structure having an internal cross-sectional area of less than one-third the aforesaid cross-sectional area of the converter vessel, means providing a path for said particulate material between said columnar structure and the converter vessel, and feeding means arranged in said path for advancing the particulate material from the columnar structure into the converter, said feeding means coacting with the gravitational advance of material in the columnar structure for controlling its downward travel in each columnar structure at a desired speed, substantially more rapid than its downward travel in the converter.

19. Apparatus as defined in claim 18, wherein the upright columnar structure comprises two vertically spaced refractory-lined columns each ladapted to be filled with the particulate material for movement downwardly by gravity, said apparatus including means providing a temperature-equalizing enclosed region between and communicating with said columns, which is substantially wider than the columns and is adapted to be filled with material moving from the upper column to the lower column, each of said columns having vertically spaced electrode means therein for passage of heating current through the body of material in the column along paths parallel to the axis thereof, said feeding means being constructed and arranged for controlling the downward travel of the material through the columns and the aforesaid intermediate equalizing zone, at the aforesaid desired speed in both of the columns.

20. Apparatus for preheating particulate metallic aluminum-containing material for continuing supply of successive quantities thereof to a converter of a system for producing purified aluminum by subhalide distillation from the Iaforesaid material, comprising vertically spaced refractory-lined columns adapted tobe lled with such particulate material for movement downwardly by gravity, means providing a temperature-equalizing enclosed region betweenand-communicating with said columns, which is adapted to be iille'dvwith material moving from the upper column to the lower column, vertically spaced electrode means in each of the columns for passing current through the body of material in the column along paths parallel to the axis thereof, and feeding means arranged at the lower end of the lower column to receive the material, coacting with the'gravitational advance of material in the columns, and adapted for communication with a converter 4to feed the material thereto, for Icontrolling the downward travel of the material at a desired rate through the columns and the aforesaid intermediate equalizing zone.

21. Apparatus as dened in claim 20, wherein the temperature-equalizing region has a substantially wider crosssectional area than both of the columns, so that on downwardk travel ofthe material at said desired rate under control of the feeding means, the particles of said material move down at .a substantially slower speed in said region than in the columns, and are subjected to lateral movement in said region, for there distributing heat among the particles.

No references cited.

DAVID L. RECK, Primary Examiner. H. W. TARRING, Assistant Examiner. 

1. A METHOD OF HEATING A CONTINUING FEED OF A MASS WHICH IS COMPOSED OF PARTICULATE, SOLID METALLIC MATERIAL AND WHICH HAS A NEGATIVE TEMPERATURE COEFFICIENT OF RESISTIVITY OVER A RANGE OF ELEVATED TEMPERATURES, FOR RAISING THE TEMPERATURE OF SAID PARTICULATE MATERIAL TO A PREDETERMINED VALUE AT LEAST SUBSTANTIALLY HIGHER THAN THE LOWER LIMIT OF SAID RANGE, FROM A VALUE SUBSTANTIALLY BELOW SAID PREDETERMINED VALUE, COMPRISING ADVANCING SAID MASS OF METALLIC PARTICLES ALONG A PREDETERMINDED DOWNWARD PATH WHILE PASSING ELECTRIC CURRENT THROUGH SAID MASS TO GENERATE HEAT THEREIN, SAID ADVANCE OF THE MASS OF PARTICLES INCLUDING IMPARTING PULSATIONS OF MOVEMENT, THROUGH THE MASS, WHICH PRODUCE MUTUAL DISPLACEMENT OF INDIVIDIUAL PARTICLES, AND SAID PULSATIONS BEING EFFECTED WITH SUFICIENT FREQUENCY TO PREVENT SUBSTANTIAL MUTUAL ADHESION OF PARTICLES IN THE MASS BY FUSION UNDER THE HEATING INFLUENCE OF THE CURRENT, SAID MASS BEING ADVANCED BY MOVING IT UNDER GRAVITY ALONG SAID DOWNWARD PATH IN A CONFINED, COLUMNAR REGION, SAID DOWNWARD MOVEMENT BEING EFFECTUATED WITH THE AFORESAID PULSATIONS BY DISCHARGING THE MATERIAL OF THE MASS AT A LOCALITY BELOW SAID REGION AT A RATE SELECTED TO IMPART SAID PULSATIONS WITH THE AFORESAID SUFFICIENT FREQUENCY.
 4. IN A PROCESS FOR SEPARATION OF ALUMINUM BY SUBHALIDE DISTILLATION AT A PREDETERMINED ELEVATED TEMPERATURE FROM MASSES WHICH ARECOMPOSED OF PARTICULATE, SOLID METALLIC MATERIAL CONTAINING ALUMINUM AND OTHER METAL AND WHICH HAVE A NEGATIVE TEMPERATURE COEFFICIENT OF RESISTIVITY OVER A RANGE OF ELEVATED TEMPERATURE, WHEREIN SAID SAID PREDETERMINED ELEVATED TEMPERATURE, WHEREIN SAID MATERIAL IS TREATED FOR CONVERSION OF ALUMINUM THEREIN TO ALUMINUM SUBHALIDE, BY ADVANCING THE MATERIAL THROUGH AN ENCLOSED ZONE WHILE HEAT ENERGY IS THERE SUPPLIED TO THE MATERIAL AND WHILE GASEOUS HALIDE IS THERE PASSED IN CONTACT WITH THE MATERIAL FOR CONVERSION REACTION WITH THE ALUMINUM TO PRODUCE ALUMINUM SUBHALIDE IN GASEOUS FORM; THE METHOD OF PREHEATING SAID PARTICULATE METALLIC MATERIAL WHILE ADVANCING THE SAME TO SAID CONVERSION ZONE, WHICH INCLUDES ADVANCING SAID PARTICULATE MATERIAL AS A MASS THEREOF ALONG A PREDETERMINED PATH WHILE PASSING ELECTRIC CURRENT THROUGH SAID LAST-MENTIONED MASS TO GENERATE HEAT THEREIN TO RAISE THE TEMPERATURE OF SAID MASS TO A VALUE SUBSTANTIALLY ABOVE THE LOWER LIMIT OF THE AFORESAID RANGE OF ELEVATED TEMPERATURES, SAID ADVANCING OF THE MASS OF PARTICLES INCLUDING IMPARTING PULSATIONS OF MOVEMENT THROUGH THE MASS WHICH PRODUCE MUTUAL DISPLACEMENT OF INDIVIDUAL PARTICLES, AND SAID PULSATIONS BEING EFFECTED WITH SUFFICIENT FREQUENCY TO PREVENT SUBSTANTIAL FUSION OF PARTICLES TO EACH OTHER UNDER THE HEATING INFLUENCE OF THE CURRENT.
 16. IN APPARATUS FOR PRODUCING PURIFIED ALUMINUM BY SUBHALIDE DISTILLATION FROM PARTICULATE METALLIC ALUMINUMCONTAINING MATERIAL, IN COMBINATION, A CONVERTER COMPRISING AN UPRIGHT COLUMNAR VESSEL ADAPTED TO RECEIVE A CONTINUING DOWNWARDLY TRAVELING MASS OF SAID MATERIAL, HAVING VERTICALLY SPACED ELECTRODE MEANS FOR PASSAGE OF CURRENT THROUGH THE MATERIAL IN THE VESSEL FOR SUPPLY OF HEAT THEREIN, SAID VESSEL HAVING GAS INLET AND OUTLET MEANS AT RESPECTIVELY OPPOSITE ENDS THEREOF, AND HAVING A PREDETERMINED INTERNAL CROSS-SECTIONAL AREA, AND MEANS FOR PREHEATING THE PARTICULATE MATERIAL OR SUPPLY INTO SAID CONVERTER, COMPRISING UPRIGHT COLUMNAR STRUCTURE ABOVE THE CONVERTER VESSEL, TO RECEIVE A CONTINUING MASS OF SAID PARTICULATE MATERIAL FORDOWNWARD ADVANCE, VERTICALLY SPACED ELECTRODE MEANS IN SAID COLUMNAR STRUCTURE FOR PASSAGE OF CURRENT THROUGH THE MATERIAL TO PREHEAT THE SAME, SAID COLUMNAR STRUCTURE HAVING AN INTERNAL CROSSSECTIONAL AREA SUBSTANTIALLY SMALLER THAN THE AFORESAID CROSS-SECTIONAL AREA OF THE CONVERTER VESSEL, TO PROVIDE SUBSTANTIALLY MORE RAPID DOWNWARD ADVANCE OF THE PARTICULATE MASS IN THE COLUMNAR STRUCTURE THAN IN THE CONVERTER, AND MEANS PROVIDING A PATH FOR SAID PARTICULATE MATERIAL BETWEEN SAID COLUMNAR STRUCTURE AND THE CONVERTER VESSEL. 